This invention is directed to heat curable protective coatings and more particularly to a sulfonic acid functional polymer adapted to be crosslinked with an amine polymer.
This invention pertains to protective surface coatings commonly known as paint coatings. Protective or surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as a dispersant for the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Polymeric binders can be thermoplastic or thermosetting binders based on coreactive components such as a reactive functional polymer adapted to crosslink or coreact with a crosslinking component such as melamine or isocyanate.
Conventional thermosetting polymers often require high temperatures as well as external crosslinkers. Some crosslinkers, such as melamines in conventional industrial coatings or triglycidyl isocyanurate for powder coatings, can cause toxicity problems. Also, the release of volatile by-products, such as caprolactam, from some of these materials can cause film defects, such as cratering and bubbling.
It now has been found that excellent non-aqueous and aqueous paint coatings can be produced based on an polymeric binder comprising a sulfonic acid functional polymer coreacted or neutralized with an amine functional polymer. Blocking of the sulfonic acid group on the polymer with a volatile amine prevents intermolecular association until heat curing (baking). Thus, sulfonic acid functional ionic polymers can be crosslinked with amine functional crosslinkers such as diamines or oligomeric amines. The ionic polymers or "polysalts" were found to provide interreacting polymer chains which exhibit thermosetting properties. A polysalt can be defined as a polymer composed of a polymeric backbone containing a small amount of pendant acid groups, and in this invention usually less than 30 mole percent sulfonic acid groups, which are neutralized partially or completely with an amine functional oligomer or polymer. These ionic moieties and their interactions dominate the behavior of the polymer itself where the amine functional polymers underdo an acid-base reaction with the sulfonic acid functional polymer. The sulfonic acid group loses its proton to the amine group and hence, an ion pair is formed. A highly desirable polymer can contain both sulfonic acid and amine functionality whereby the polymer becomes self crosslinking upon heat curing.
Ionic hydrocarbon polymers for elastomers or plastics based on carboxyl functional polymers are disclosed in U.S. Pat. No. 3,264,272. Related copending and commonly assigned applications for patent are Ser. No. 423,050 filed Oct. 18, 1989 pertaining to carboxyl functional latex binders neutralized with a zinc salt; Ser. No. 397,280 filed Aug. 23, 1989, pertaining to certain carboxyl functional polymers for paint coating neutralized with an organic zinc compound; Ser. No. 397,279 filed Aug. 23, 1989 pertaining to certain carboxyl functional polymers for paint coatings neutralized with zinc carbonate; and Ser. No. 424,981 filed Oct. 23, 1989 pertaining to carboxyl functional latex binders neutralized with a zinc salt.
In accordance with this invention, it has been found that changes in the properties of polymeric binders for paint coatings can be achieved through the introduction of amine functional crosslinkers where modification of polymer properties is believed partially due to ion pairs formed from adjacent polymer chains which behaves essentially like a crosslink. Among the dramatic effects that have been observed with paint compositions are increases in the moduli, increases in glass transition temperatures, and increases in viscosities. The formation of ionomeric clusters in protective surface coatings causes the sulfonic acid functional polymers to behave like a phase-separated block copolymer. Being that a sulfonic acid group is a stronger acid than a carboxyl acid group, the driving force to form the ionic spcies in the presence of amine functionality is greater. Coatings based on the resulting sulfonic acid polysalts exhibit excellent film properties. The use of polysalts in powder coatings specifically is particularly advantageous since control of the process is such that the temperature during extrusion is lower than that of the volatilization temperature of the blocked sulfonic acid group. Thus, the "crosslinks" are not formed until after processing of the powder coating, application to a substrate, melting of the powder, flow out and leveling to form a smooth continuous film. In this invention, the actual crosslinking takes place during the bake cycle which comprises cure temperatures higher than the temperature where the sulfonic acid group becomes unblocked. It has been found that sulfonic acid acrylic polymeric binders are more reactive than similar carboxylic acid functional polymer, and consequently, the cures are faster and at lower temperatures to provide improved cured films exhibiting improved properties such as solvent resistance and hardness.
In essence, the sulfonic acid polymers can be blended with amine, pyridine, amidine, and similar nitrogen containing functional compounds or polymers to give a system containing mixed ionizable functionalities. The sulfonic acid group, in this case, could be blocked using a suitable blocking group, such as an amine. The blocking group volatilizes during the cure cycle, and the two polymers coreact by an acid-base reaction to form an ionic crosslinked matrix. The polymer network formed exhibits solvent resistance, high gloss, and hardness.
These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.